Thomas & Friends: The Mystery of Lookout Mountain

Thomas and Percy travel into an old mine on Sodor to collect rails for Whiff and dirt carts for McColl's Farm. Inside, the cavern rumbles, and one of Percy's dirt carts disappears from view. When Percy fears it was taken by a creature, Thomas calls out, only for an unfamiliar voice in the darkness to answer, "Who's there?" latestnetflix.com's read is that a mechanical whirring sound echoes through the tunnel, the missing carts roll back out on their own, and both engines flee the mine.

At the maintenance yard, Thomas and Percy recount the rumbling and vanishing carts to Diesel, Carly, Sandy, Nia, and Bruno. Diesel delivers a load of scrap, where an old metal sign bearing a red cross jams the machinery. Bruno inspects the sign and states, "Danger."

Whiff arrives and recognizes the sign from an abandoned site, telling the engines, "There used to be a mine under Lookout Mountain!" He explains that older tunnels beneath the mountain were shut down when support beams failed, after which mysterious clanking and vanishing carts began occurring. Although Percy and Diesel suspect a subterranean creature, Bruno insists, "I do not believe in Mine Monsters."

The engines organize themselves as the Biggest Adventure Club to investigate the connection between the two locations. Thomas, Percy, Sandy, and Bruno form the Lookout Mountaineers to search the surface of Lookout Mountain, while Diesel, Kana, Carly, and Nia head into the old mine as the Cave Crawlers.

On the mountain slopes, Percy parks the club's equipment cart, the Biggest Adventure Machine, but discovers it has vanished after a heavy vibration shakes the ground. While taking a rest near a river, Bruno notices a set of rails running behind the water and asks, "Tracks under the waterfall?" Realizing this is an unmapped entrance into Lookout Mountain, the four engines venture inside.

Meanwhile, the Cave Crawlers enter the old mine and find Percy's missing dirt carts just before another tremor sends the carts rolling into the depths. Following the path downward, the group discovers crushed carts, scrape marks across the rock walls, and a Troublesome Truck that derailed into a newly exposed tunnel.

Inside the waterfall passage, the Lookout Mountaineers navigate a large cavern filled with fossils. To prevent getting lost along the branching routes, Sandy uses her tools to etch symbols onto the rock at each switch point, leaving markers such as a pickaxe and a wrench to trace their return.

Deeper underground, the runaway Biggest Adventure Machine hurtles down the steep tracks with its siren wailing. The Cave Crawlers follow the slope at high speed until both search parties abruptly slide into a cavern containing a massive central turntable.

The siren of the missing machine echoes from multiple directions across the cavern walls, confusing the search party. Bruno warns the group, "You are chasing echoes." When the chamber lights fail, Nia points out that the machine's warning lights are flashing, allowing the engines to track the vehicle's illumination rather than its sound.

Following the glow into a newly excavated tunnel, the engines locate the gathered mine carts and the missing machine. A sudden grinding noise breaks through the rock wall, revealing a large mechanical drill that prompts the panicked engines to scream, "Mine Monster!" and speed back toward the exit.

The engines race upward along the path Sandy marked with her tools, passing the wrench and pickaxe signs until they burst out onto the surface of Lookout Mountain. The drilling machine follows them into the daylight. Bruno examines the newcomer and clarifies, "This is not a Mine Monster. This is a mine drill."

The machine introduces herself, announcing, "Yup! I'm Darcy. Darcy the Driller!" She explains that she has been drilling the tunnels beneath the mountain alone, using the mine carts to haul away dirt and rubble before launching them back up when full. She reveals that the weak wooden support beams kept collapsing, leaving her work unfinished.

The engines accompany Darcy down the mountain to Whiff's recycling plant. Whiff provides a supply of recycled metal support beams strong enough to stabilize the underground tunnels. Percy collects his loaded dirt carts to complete his delivery to McColl's Farm, and the engines return to help Darcy reinforce the mountain shafts.

With the subterranean passages secured, Sir Topham Hatt and Emily meet the engines at the mountain. Bruno shares the full account of the investigation, and a commemorative club flag is presented to celebrate Darcy and the team's shared adventure.